Homologous recombination (HR) represents a major, evolutionarily conserved chromosome damage repair tool. By eliminating radiation-induced DNA double-strand breaks and other deleterious chromosome lesions, HR is critical for the maintenance of genome stability. In humans, defects in HR directly lead to the cancer phenotype. This renewal research project focuses on the roles that two tumor suppressors, BRCA2 and PALB2, and their partner proteins fulfill in the HR-mediated repair of damaged chromosomes. Considerable progress has been made during the last funding period, leading to important advances in understanding HR mechanism. Capitalizing on our success, a variety of in vitro and in vivo studies will be conducted to delineate the multi- faceted roles of the BRCA2-PALB2 complex and associated proteins in the HR reaction. Novel hypotheses regarding how the HR factors facilitate the presynaptic and synaptic stages of the HR reaction will be tested. We fully anticipate the results from this renewal project to continue providing insights regarding the functions of key HR factors, and to allow us to formulate detailed models of the BRCA2-PALB2 axis of chromosome damage repair in human cells. Given the importance of HR in tumor suppression and in the removal of DNA crosslinks induced by chemotherapeutic agents, our studies have direct relevance to cancer biology and to the development of molecules to evaluate the potential of HR pathway-targeted therapeutic strategies. Moreover, the research materials generated during the course of our studies represent an invaluable resource for the DNA repair and cancer biology research communities. PUBLIC HEALTH RELEVANCE: Failure to repair damaged chromosomes compromises the integrity of the genome and can lead to cancer formation. The proposed studies will delineate the role of two well- known tumor suppressors and their partner proteins in the homologous recombinational repair of chromosomes that harbor radiation-induced double-strand breaks and other deleterious lesions. The results will make a major contribution toward delineating the mechanism of HR in human cells and have direct relevance to understanding the role of DNA repair in cancer avoidance.